In recent years, detection of gas in the environment has become increasingly important for avoidance of danger caused by noxious gas, odorless carbon monoxide gas and the like in the environment. It is also known that a specific gas is developed due to cancer present in the body, an etiology such as diabetes and the like, and the application of a gas sensor to a noncontact diagnosis using mouth cavity gas is expected. In addition, a gas sensor which is compact and light weight, low power consuming and superior in a gas type discriminating function (property to sense a particular gas component) is desired as an instrument to be mounted on a disaster robot.
Examples of the gas sensor commercially available heretofore include a PN junction type gas sensor, a metal oxide sintered gas sensor and the like. In recent years, a gas sensor using a single-walled carbon nanotube (SWCNT) (non-patent documents 1, 2), and a gas sensor using a metal oxide nanowire (non-patent document 3) have been proposed. In addition, the applicant of the present application proposed gas sensor using a selenium nanowire (SeNW) (patent document 1). However, these gas sensors are not very good at discriminating gas types (hereinafter to be also referred to as “gas type discriminating ability”). That is, it is difficult to distinguish a gas showing similar reactivity (sensitivity) with a gas sensitive material, and the gas sensor is limitatively used under conditions for a predetermined gas type to be detected.
As a method of improving gas type discriminating ability of a gas sensor, an apparatus accumulating gas sensors has been considered. For example, patent document 2 proposes a gas sensor array comprising a combination of a semiconductor gas sensor use Cr(2-x)TixO3 (wherein 0.3≧x≧0.05) as a gas detection element, and a semiconductor gas sensor using Fe(1-x)BxNbO4 (B is a trivalent element and 1≧x≧0), CrNbO4 or SnO2 as a gas detection element. Patent document 3 proposes a gas measurement substrate unit wherein a plurality of gas adsorption films having different gas adsorption properties are accumulated on a single crystal silicon substrate. In addition, patent document 4 proposes a gas sensor array using a plurality of conductive polymer films having different gas responsiveness. However, all of these have plural gas-sensitive parts formed from materials showing different gas responsiveness, and the formation steps thereof are highly complicated. Moreover, the number of gases that can be determined is limited since it depends on the number of materials used for the gas-sensitive part. In the case of the gas sensor arrays of patent documents 3, 4, gas detection requires a detection operation at a high temperature using a heater, which consumes a large amount of electric power.
For the component analysis of a mixed gas, gas chromatography is generally used, and there is no report on the component analysis of a mixed gas by a gas sensor array accumulating gas sensors as described in patent documents 2-4. A gas sensor as semiconductor gas chromatography can only distinguish gases such as hydrogen, acetylene, ethane and the like and cannot easily distinguish various organic gases. In addition, since a metal oxide semiconductor is used for semiconductor gas chromatography, oxidation heating by a heater is necessary, thus causing high power consumption. Furthermore, since such gas chromatography requires time for analysis measurement and has a bulky apparatus unsuitable for carrying, it is not suitable for environmental monitoring at the work site.